1. Field of the Invention
This invention relates to a transient, rapid spectroscopic method for analysis of unknown heterogeneous materials by plasma spectroscopy.
2. Related Art
Most analytical techniques used in industry require taking samples to the laboratory, to be analyzed by time consuming procedures involving instrumentation such as Auger and mass spectrometers, EDS, liquid or gas chromatography, graphite furnace atomic absorption spectroscopy or inductively coupled plasma optical emission spectrometry. Faster in-situ methods such as spark-discharge optical spectrometry are only applicable to electrically conductive materials, while X-ray backscattering probes are limited in sensitivity.
An emerging method, laser induced plasma spectroscopy, promises to provide rapid, in-situ compositional analysis of a variety of materials in hostile environments and at a distance. Basically, this method includes focusing a high power pulsed laser on the material, thus vaporizing and ionizing a small volume of the material to produce a plasma having an elemental composition which is representative of the composition of the material. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition. This method has been applied to a variety of materials and industrial environments, as exemplified in the following documents.
U.S. Pat. No. 4,645,342 in the name of Tanimoto et al. describes a probe for spectroscopic analysis of steel including focusing an infrared laser pulse on the steel material and collecting, at an angle of 16 degrees or more, the light emitted by the irradiated surface spot, such light being spectrally analyzed. The continuous spectral background is subtracted to obtain the net intensity of preset spectral lines representative of given elements, and the intensity of said spectral lines is related to the concentration of said elements in the steel material. The probe includes a single laser pulse not collinear with the collecting optics.
U.S. Pat. No. 4,986,658 to Kim describes a probe for molten metal analysis by laser induced plasma spectroscopy. The probe contains a high-power laser producing a pulse having a triangular pulse waveshape. When the probe head is immersed in the molten metal, the pulsed laser beam vaporizes a portion of the molten metal to produce a plasma having an elemental composition representative of the molten metal composition. Within the probe, there is provided a pair of spectrographs each having a diffraction grating coupled to a gated intensified photodiode array. The spectroscopic atomic emission of the plasma is analyzed and detected for two separate time windows during the life of the plasma using two spectrometers in parallel. The first time window analyzes the plasma plume before it reaches thermal equilibrium shortly after the termination of the laser pulse, typically 10 nanoseconds long, to detect line reversals caused by absorption of radiation emitted by the hotter inner portion of the plasma plume by relatively cooler outer portions of the plasma plume. Thereafter, after the plasma has reached thermal equilibrium, typically after 1 microsecond, a second time window analyzes the more conventional line emissions from the optically emissive plasma. The spectra obtained during either the first or the second time window, or a combination of both, can be used to infer the atomic composition of the molten metal. In this case the vaporizing laser beam and collecting optics are collinear, but only a single laser pulse is used to vaporize the molten metal surface.
U.S. Pat. No. 5,042,947 to Potzschke et al. describes an application of laser induced plasma spectroscopy for the sorting of solid metal particles, namely shredder scrap from automotive recycling processes. Multiple laser pulses are used to clean the surface from impurities, and up to 30 particles per second can thus be sorted depending on the resulting composition, typically aluminum, zinc, copper, lead and steel. Because the purpose is sorting rather than precise compositional analysis, a relatively low precision and sensitivity can be accepted. A single laser pulse is used to produce each laser spark.
U.S. Pat. No. 5,379,103 to Zigler describes a mobile laboratory for in-situ detection of organic and heavy metal pollutants in ground water. Pulsed laser energy is delivered via fiber optic media to create a laser spark on a remotely located analysis sample. The system operates in two modes, one being based on laser induced plasma spectroscopy and the other on laser induced fluorescence. In the first operational mode, the laser beam guided by fiber optics is focused by a lens on the sample to generate a plasma. The emitted spectrum is analyzed and used to detect heavy metals. In the second mode the focusing laser energy is removed allowing the laser beam via fiber optics to irradiate the sample, so that organic molecules with an aromatic structure emit absorbed ultraviolet energy as fluorescence. The emitted fluorescence light is transmitted via fiber optic media for further analysis. The measured wavelength and time characteristics of the emitted fluorescence can be compared against predetermined characteristics to identify the organic substances in the analysis sample. Again, only single laser pulses are used to analyze pollutants in ground water.
U.S. Pat. No. 5,757,484 by Miles et al. describes a subsurface soil contaminant identification system using a cone penetrometer and based on laser induced breakdown spectrometry. An optical fiber link is used to couple the transmitted and collected beams through the penetrometer to the subsurface soil region to be analyzed. In all of the above patents, single laser pulses are used to vaporize, ionize and excite a portion of the material to be analyzed by laser induced plasma emission spectroscopy.
Piepmeier et al., Anal. Chem. 41, 700 (1969), discuss their work on multiple laser pulses for time and spatially resolved spectrometric observations of laser generated plumes from the surface of a solid aluminum alloy. The laser pulse sequences are random and cannot be controlled, therefore this work does not offer much assistance in quantitative spectrometric analysis.
Two temporally close sparks induced by two collinear lasers are used by Cremers et al. in U.S. Pat. No. 4,925,307 for the spectrochemical analysis of liquids. The laser light is not significantly absorbed by the sample so that the sparks occur in the volume inside the liquid. The spark produced by the first laser pulse produces a bubble in the liquid which stays in the gaseous state for hundreds of microseconds after the first spark has decayed, so that the second laser pulse, fired typically 18 microseconds after the first pulse, will produce a second spark within the gaseous bubble. The spark generated inside the cavity has excitation characteristics close to those of a non-ionized air spark. Due to the much lower density of the gas compared to the liquid, the temperature of the second spark, produced within the gas bubble, is much higher than the temperature of the first spark produced within the liquid. The emission spectrum of the second spark, detected by a spectrometer oriented at 90 degrees from the laser beam axis, is thus much more intense and exhibits reduced line widths compared to the first spark, so that a much increased detectability of the atomic species is obtained by sampling the bubble with the second laser spark. This approach is convenient to analyze relatively transparent liquids so that the first spark is produced in the bulk of the liquid, while the second spark is obtained after a convenient period of time, typically 18 microseconds, has elapsed so that the gas bubble produced by the first pulse has had the time to expand and stabilize. During this period of time the atomic or molecular components in the gas bubble become de-excited and cool down, for optimal line width and detectability. The second pulse thus substantially re-excites a relatively cool gas within a liquid-surrounded bubble.